Electronic circuit for converting a differential signal into a single-ended signal with common mode voltage rejection by resistor network

ABSTRACT

A differential-to-single-ended converter comprising a resistor network (205) and an operational amplifier is introduced. In comparison to prior art converters, a resistor (250) placed between the non-inverting input (264) of the operational amplifier (260) and the negative input terminal (202) of the converter (200). The common mode voltage (V nii  &#39;) at the non-inverting input (264) does not depend on the differential input voltage (V in   # ) of the converter (200) and has low fluctuations. This allows the use of an operational amplifier (260) with low CMRR and makes the converter (200) suitable for low voltage applications.

FIELD OF THE INVENTION

This invention relates to semiconductor electronic devices, especially to differential-to-single-ended converters.

BACKGROUND OF THE INVENTION

Analog portions of mixed integrated circuits are often fully differential but have a single-ended output. A differential-to-single-ended converter, hereinafter `converter` is an electronic circuit transforming a double path input signal into a single-ended output signal.

FIG. 1 is a simplified circuit diagram of converter 100 as known in prior art.

Converter 100 comprises resistors R₁, R₂, R₃, R₄ (110, 120, 130, 140) and operational amplifier 160.

Resistor R₁ (110) is coupled between negative input terminal 102 and inverting input 162 of operational amplifier 160. Resistor R₂ (120) is coupled between inverting input 162 and output 166 of operational amplifier 160. Output 166 is coupled to output terminal 106 of converter 100. Resistor R₃ (130) is coupled between positive input terminal 104 and non-inverting input 164. Resistor R₄ (140) is coupled between non-inverting input 164 and reference terminal 109 (e.g., ground).

Signals and their symbols are introduced for further explanations. Unless otherwise specified, all voltages are referred to a reference terminal 109 (e.g., ground). Converter 100 receives input voltages V_(p) and V_(n) at input terminals 104, 102 and supplies output voltage V_(out) to output terminal 106.

Input voltages V_(p) and V_(n) have a common mode component V_(pn) ' and differential mode components V_(p) ^(#), V_(n) ^(#). They are related as V_(p) =V_(pn) '+V_(p) ^(#) and V_(n) =V_(pn) '+V_(n) ^(#). V_(in) ^(#) is the differential input voltage V_(in) ^(#) =V_(p) -V_(n) =V_(p) ^(#) -V_(n) ^(#).

Output voltage V_(out) =V_(out) '+V_(out) ^(#) has a DC component V_(out) ' and an AC component V_(out) ^(#).

At operational amplifier 160, V_(ni) and V_(i) are the voltages at non-inverting input 164 and inverting input 162, respectively. V_(ni) and V_(i) have common and differential mode components: V_(ni) =V_(nii) '+V_(ni) ^(#) and V_(i) =V_(nii) '+V_(i) ^(#).

In converter 100, input voltages V_(p), V_(n) are transferred into output voltage V_(out). The information signal exists only in the differential or AC voltages (V_(p) ^(#), V_(n) ^(#), V_(in) ^(#), V_(out) ^(#)), whereas common mode or DC voltages (V_(pn) ', V_(nii) ', V_(out) ') contribute to unwanted effects such as for example noise or bandwidth limitation.

Converter 100 has differential gain A^(#) =V_(out) ^(#) /V_(in) ^(#) which is desired to be linear. Converter 100 also has a common mode rejection ratio CMRR=V_(pn) '/V_(out) ' (also CMRR=ΔV_(pn) '/ΔV_(out) ') which should be maximized.

If the resistors are in the relation R₁ /R₂ =R₃ /R₄, the common mode voltage V_(nii) ' at non-inverting input 164 does not affect the DC output voltage V_(out) '. In this case, a change ΔV_(nii) ' would also have no influence on V_(out) '. However, the common mode voltage V_(nii) ' is related to the differential mode input voltage V_(in) ^(#). The common mode voltage V_(nii) ' at operational amplifier 160 changes even if the common mode input voltage V_(pn) ' at input terminals 104, 102 is constant.

Operational amplifier 160 itself can reject the common mode voltage V_(nii) ' and changes ΔV_(nii) ' therein so that the DC output voltage V_(out) ' remains unchanged. But this feature is limited by the common-mode rejection ratio (CMRR) of operational amplifier 160. The CMRR can depend on changes of V_(nii) '. Constructing operational amplifiers 160 having sufficient high CMRR is expensive and not always possible.

The common mode voltage V_(nii) ' can be limited by the choice of the resistors R₁, R₂, R₃, R₄. However, higher common mode rejection demands higher resistor ratios (R₁ /R₂) making converter 100 more sensitive to noise.

It is also difficult to provide operational amplifier 160 with high CMRR values at medium and high signal frequencies of e.g. 0.5 MHz and above. At still higher frequencies, common-mode capacities at the input of converter 100 become influential, further degrading performance.

The capability of operational amplifier 160 to accommodate a common mode voltage V_(nii) ' in a large range is limited by the supply voltage. That makes it more difficult to use prior art converter 100 in modem low voltage circuits.

The mentioned problems can lead to the consequence that converter 100 must eventually be a stand-alone circuit and can not be integrated into a larger signal processing chip. This is a typical situation for the most popular designs of operational amplifiers 160 and converters 100.

With prior art converter 100, it is also difficult to optimize different factors such as CMRR, resistor values, noise, bandwidth, feedback loop depth and others.

Accordingly, there is an ongoing need to provide a converter which overcomes some or all the deficiencies of the prior art.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. I is a simplified circuit diagram of a differential-to-single-ended converter according to the prior art; and

FIG. 2 is a simplified circuit diagram of a differential-to-single-ended converter in a preferred embodiment of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 2 is a simplified circuit diagram of electronic circuit 200 according to the present invention. Electronic circuit 200, hereinafter converter 200, converts a differential signal into a single-ended signal. Converter 200 comprises resistor network 205 and operational amplifier 260. Resistor network 205 comprises resistors R₁, R₂, R₃, R₄, R₅ (210, 220, 230, 240, 250). As used therein, the word `resistor` is intended to indicate any element exhibiting resistance, inductance, and capacitance.

Resistor R₁ (210) is coupled between negative input terminal 202 and inverting input 262 of operational amplifier 260. Resistor R₂ (220) is coupled between inverting input 262 and output 266 of operational amplifier 260. Output 266 is coupled to output terminal 206 of converter 200. Resistor R₃ (230) is coupled between positive input terminal 204 and at node 265 to non-inverting input 264. Resistor R₄ (240) is coupled between node 265 at non-inverting input 264 and reference terminal 209. Resistor R₅ (250) is coupled between negative input terminal 202 and node 265 at non-inverting input 264.

The voltages are defined in the same way as for prior art converter 100. Unless otherwise specified, all voltages are referred to reference terminal 209 (e.g., ground).

Converter 200 receives input voltages V_(p) and V_(n) at input terminal 204, 202 and supplies output voltage V_(out) to output terminal 206.

Input voltages V_(p), V_(n) have a common mode component V_(pn) ' and differential mode components V_(p) ^(#), V_(n) ^(#). They are related as V_(p) =V_(pn) '+V_(p) ^(#) and V_(n) =V_(pn) '+V_(n) ^(#). V_(in) ^(#) is the differential input voltage V_(in) ^(#) =V_(p) -V_(n) =V_(p) ^(#) -V_(n) ^(#).

Output voltage V_(out) has a DC component V_(out) ' and an AC component V_(AC) : V_(out) =V_(out) '+V_(out) ^(#).

At operational amplifier 260, V_(ni) and V_(i) are the voltages at non-inverting input 264 and inverting input 262, respectively. V_(ni) and V_(i) have common and differential mode components: V_(ni) =V_(nii) '+V_(ni) ^(#) and Vi=V_(nii) '+V_(i) ^(#).

In comparison to converter 100 of prior art, converter 200 comprises resistor R₅. The values of resistor R₃ and resistor R₅ are substantially equal.

V_(p) relates to V_(ni) as:

    V.sub.ni =V.sub.p *R.sub.4 /(R.sub.3 +R.sub.4)             (1)

V_(ni) and V_(n) relate to V_(out) as:

    V.sub.out -V.sub.ni =(V.sub.n -V.sub.ni)*(-R.sub.2 /R.sub.1)(2)

    V.sub.out =V.sub.ni *(R.sub.2 /R.sub.1 +1)-V.sub.n *R.sub.2 /R.sub.1(3)

For the common mode voltage V_(pn) ' it can be considered that R₃ is parallel coupled to R₅. Therefore, the common mode voltage V_(nii) ' equals:

    V.sub.nii '=V.sub.pn '*R.sub.4 /(R.sub.3 #R.sub.5 +R.sub.4)(4)

R₃ #R₅ is the abbreviation for R₃ *R₅ /(R₃ +R₅).

Equations (3) and (4) are combined and considered for common mode only:

    V.sub.out '=V.sub.pn '* (R.sub.4 /(R.sub.3 #R.sub.5 +R.sub.4))*(R.sub.2 /R.sub.1 +1)-R.sub.2 /R.sub.1 !                           (5)

The DC voltage V_(out) ' at output 266 becomes zero under the condition:

    R.sub.4 /(R.sub.3 #R.sub.5 +R.sub.4)*(R.sub.2 /R.sub.1 +1)=R.sub.2 /R.sub.1(6)

when R₃ =R₅, the condition (6) can be simplified to:

    R.sub.1 /R.sub.2 =R.sub.3 /(2*R.sub.4)                     (7)

The ratio of the value of resistor R₁ (210) to the value of resistor R₂ (220) substantially equals the ratio of the value of resistor R₃ (230) to twice the value of resistor R₄ (240). If the condition (7) is satisfied, common mode fluctuations ΔV_(pn) ' of the input voltages V_(p) and V_(n) do not affect the output voltage V_(out).

In comparison to prior art, the common mode voltage V_(nii) ' at non-inverting input 264 is not related to the differential input voltage V_(in) ^(#). That means that changes of the input voltage V_(in) ^(#) (ΔV_(n) ^(#) =-ΔV_(p) ^(#)) do not change the common mode component V_(nii) ' at non-inverting input 264. If the voltage V_(nii) ' is constant then operational amplifier 260 can have a lower common mode impedance than an operational amplifier where V_(nii) ' is fluctuating. In other words, an operational amplifier 160 as used in prior art converter 100 must be able to receive a fluctuating voltage V_(nii) '. The supply voltage for operational amplifier 160, 260 must be higher than the range of V_(nii) '. In converter 200 of the invention, the range of V_(nii) ' can substantially be reduced by resistor network 205. Therefore, the supply voltage for operational amplifier 260 can also be reduced. Converter 200 is therefore suitable for low voltage applications.

The voltage V_(nii) ' at non-inverting input 264 is held at a constant value. Differential voltages V_(in) ^(#) are amplified to AC output voltage V_(out) ^(#). The gain is A=R₂ /R₁.

It will be appreciated that although only one particular embodiment of the invention has been described in detail, various modifications and improvements can be made by a person skilled in the art based on the teachings herein without departing from the scope of the present invention.

For example, it is possible to replace resistors R₁, R₂, R₃, R₄, R₅ by complex resistors which have real and imaginary components. Such complex resistors can be, e.g., capacitors, coils, other components or a combination thereof. In that case, converter 200 can have additional functions, such as for example, the function of a filter.

Having thus described the invention it will be apparent that a converter according to the invention comprises an additional resistor R₅ coupled between negative input terminal and non-inverting input. The converter of the invention has all properties of the prior art plus additional advantages. For example, the common mode voltage V_(nii) ' at the input of the operational amplifier is not related to the differential input voltage V_(in) ^(#). Further, the requirements to the CMRR of the operational amplifier can be reduced. This allows the cost-efficient use of operational amplifiers with low CMRR and low common mode impedance in circuits where previously expensive operational amplifiers had to be used.

Accordingly, the converter of the invention is especially suitable for low voltage applications. 

We claim:
 1. An electronic circuit for converting a differential input voltage to a single-ended output voltage havingan operational amplifier; a resistor R₁ coupled between a negative input terminal and an inverting input of said operational amplifier; a resistor R₂ coupled between the inverting input and an output of said operational amplifier; a resistor R₃ coupled between a positive input terminal and a non-inverting input of said operational amplifier; and a resistor R₄ coupled between the non-inverting input of said operational amplifier and a reference terminal; characterized in that said electronic circuit further comprises: a resistor R₅ coupled between the negative input terminal and the non-inverting input of said operational amplifier, by-passing said resistor R₁ R₃.
 2. The electronic circuit of claim 1,characterized in that the values of said resistor R₃ and said resistor R₅ are substantially equal.
 3. The electronic circuit of claim 1,characterized in that said resistor R₁ has a first value andsaid resistor R₂ has a second value and said resistor R₃ has a third value and said resistor R₄ has a fourth value, and whereina ratio of said first value to said second value substantially equals a ratio of said third value to twice said fourth value.
 4. The electronic circuit of claim 1, wherein said resistors R₁, R₂, R₃, R₄ are substantially related in value by:

    R.sub.1 /R.sub.2 =R.sub.3 /(2*R.sub.4).


5. 5. The electronic circuit of claim 1, wherein said resistors R₃, R₅ are substantially related in value by: R₃ =R₅.
 6. The electronic circuit of claim 1, wherein said resistors R₁, R₂, R₃, R₄, R₅ are complex resistors having real and imaginary components.
 7. An electronic circuit comprising:an operational amplifier having inverting input, non-inverting input and output; a positive input terminal; a negative input terminal; an output terminal coupled to said output of said operational amplifier; a first resistor coupled between said negative input terminal and the inverting input of said operational amplifier; a second resistor coupled between the inverting input of said operational amplifier and said output; a third resistor coupled between said positive input terminal and said non-inverting input of said operational amplifier; and a fourth resistor coupled between the non-inverting input of said operational amplifier and a reference terminal characterized in that said electronic circuit further comprises a fifth resistor coupled between the negative input terminal and the non-inverting input of said operational amplifier by-passing said first and third resistors; and thatsaid third resistor and said fifth resistor have substantially equal values and that said the ratio between the values of said first resistor and said second resistor substantially equal the ratio between said third resistor and doubled a value of said fourth resistor.
 8. A resistor network as part of an electronic circuit having an operational amplifier with inverting input, non-inverting input and output, said resistor network comprising:a positive input terminal coupled by a first resistor to said non-inverting input, a negative input terminal coupled to said inverting input by a second resistor, and an output terminal coupled to said output and coupled by a third resistor to said inverting input, and a fourth resistor coupled between said non-inverting input and a reference terminal, characterized in that said network further comprisesa fifth resistor coupled between said negative input terminal and said non-inverting input by-passing the first and second resistors.
 9. The resistor network of claim 8 wherein said fifth resistor has substantially the same value as said first resistor.
 10. The resistor network of claim 8 wherein the ratio of said second resistor to said third resistor substantially equals the ratio of said first resistor to twice said fourth resistor. 